The use of membranes to effect separation of gas/gas, liquid/liquid, and liquid/solid mixtures and solutions has achieved general industrial applicability by various methods, among them being ultrafiltration, hyperfiltration, reverse osmosis, dialysis. In general, membrane elements associated with these processes are contained in vessels called modules, comprising a container having various inlet and outlet ports and an assembly of membranes within said container. The internal configurations are so arranged as to permit the introduction of a feed stream with or without pressure on the upstream face of the membranes, means for collecting permeate which passes through the membranes and emerges on their downstream faces, and means for keeping feed and permeate materials from commingling.
Membranes have been fabricated in various shapes, such as (1) flat sheets which may be supported in a typical plate and frame structure similar to a filter press; (2) flat sheets rolled into spirals with spacing materials interleaved with the membrane and the assembly sealed to provide spiroidal channels permitting the passage of a feed on one side of the coiled membrane through spaces to the opposite side of the membrane; (3) as tubes lining the inner surface of a reinforced braid, the braid itself at times being a component in a larger tube; and (4) in the form of open-ended hollow fibers so organized and sealed into header plates as to provide a separation of the flows over the external surfaces of the hollow fibers from any flow within the bores of the hollow fibers ensuing by virtue of passage of permeant across the membrane.
The subject invention is concerned with the use of hollow fibers assembled in modular form to provide the desired separation.
It has been commonly assumed that hollow fine fiber reverse osmosis modules are advantageously fabricated having an extremely high fiber packing density, combined with relatively small diameter fibers. Thus it has been considered desirable to provide a pressure vessel containing extremely high surface areas of membrane per unit volume of vessel. This feature is often presented as one of the principal areas of superiority for the hollow fiber system.
This arrangement inevitablly contributes to a number of significant problems. First, it is relatively difficult to maintain uniform, extremely close packing among cylinders such as fibers unless they are quite parallel and uniform in diameter. In conventional hollow fine fiber modules, this condition is approached but not generally fully met. As a result, there is a distribution of packing densities of the fibers within the module so that some inter-fiber capillary channels may be essentially stagnated, and in other regions fiber spacing is wide enough to permit fairly ready passage of the pressurized feed solution. As a result, during operation different regions within the module are likely to present different flow conditions to the feed solution, resulting in variable conditions of transmembrane effective pressure, concentration polarization, and susceptibility to accumulation of deposits of adventitious particles or precipitated salts from the feed stream.
The problem of membrane fouling due to retention of suspended particles is also very prominent in conventional modules. Feed streams inevitably contain varying amounts of suspended particulate matter. Although extreme precautions are generally taken to prefilter the feed stream or otherwise remove the suspended particles prior to admitting the feed to the modules, the conventional hollow fine fiber modules ultimately become effective collectors for much of this material.
Tightly packed hollow fine fiber bundles develop occluded regions and lose effective membrane areas.
Apart from adverse shellside flow considerations such as described above, it can be shown that the parasitic pressure loss due to bore flow resistance is an exponential function of both fiber length and bore diameter. With relatively low intrinsic membrane transport capabilities, fairly fine fibers can be tolerated, but only up to a limit in length. However, where intrinsic membrane permeability is high (i.e., high flux at a particular applied pressure), the fiber length and bore diameter strongly influence pressure losses which may become a major limiting factor in module productivity.
These deficiencies, among others, of conventional tightly packed fine hollow fiber modules have heretofore prevented maximum utilization of the advantages to be gained by the use of hollow fibers in separatory applications.